1. Field of the Invention
The present invention relates to a novel glycoprotein possessing activity of inhibiting complement-mediated cell membrane damage, and to a gene coding for the glycoprotein.
2. Description of the Prior Art
Complement is a reaction system which includes a group of proteases being present in blood and body fluids. In response to invasion of exogeneous (non-self) substances into the body, activation of the complement system will take place and part of complement components that have been deposited on the exogeneous substances will then actively eliminate them by, for example, facilitating phagocytosis by phagocyte as well as inducing immune cytolysis. Furthermore, various peptide fragments which have been produced in the pathways of the complement activation may act upon a variety of cells. That is, they will activate lymphocytes and promote migration of neutrophils and degranulation of mast cells, thereby taking part in diverse immune and inflammatory reactions.
Although, complement may promptly react with foreign substances, its activation does not proceed on autologous normal cells. In inflammatory parts of such diseases as rheumatoid arthritis, systemic lupus erythematosus and glomerulo nephritis, however, it is generally thought that complement recognizes a self tissue as a foreign one and reacts with cell membranes of the self tissue. Consequently, complement may induce inflammatory reaction, and at the same time it may facilitate phagocytosis by phagocytes or cause cytolysis, thereby leading to cytotoxicity and aggravation of the inflammatory condition.
In recent years, it has been shown that proteins which are capable of inhibiting self-complement activation are present on cell membrane (Mol. Immunol., 20, 1233-1236 (1983)), several of which have been identified as a protein having with this function. For example, Decay Accelerating Factor (DAF) with a molecular weight of 68 Kd (J. Exp. Med., 160, 1558-1578 (1984)) and Homologous Restriction Factor (HRF) with a molecular weight of 65 Kd (PNAS, 83, 6975-6979 (1986)) have been obtained as complement inhibiting factors present on the human cell membrane. Precise function of DAF and HRF in vivo, nevertheless, has been not yet clarified in detail.
The present inventors have studied to find a membrane protein capable of inhibiting the complement activation on the self-cell membrane, besides the above factors.
Previously, the present inventors prepared a number of antibody-producing hybridomas by hybridizing spleen cells from mice immunized with human erythrocytes with mouse myeloma cells (P3U1) in the presence of polyethyleneglycol. These hybridomas were repeatedly screened to select those producing an antibody (in a supernatant of culture medium) which causes hemolysis of neuraminidase-pretreated human erythrocytes in the presence of human complement, succeeding in cloning a hybridoma producing such monoclonal antibody, which was designated as 1F5 (Proc. Gen. Conf. Jpn. Soc. Immunol., 17, 498 (1987), and Abstracts, Symposium on Complement, 24, 180 (1987)).